Metric

Understanding Routing Metrics: What are Metrics and How are They Used?

Routing metrics are essential for determining the best path for network traffic to follow. These metrics are used to measure the cost or distance of a route and to help determine the optimal path for data to travel from one network node to another. In this blog post, we will explore what metrics are, how they are used, and how they are calculated, with a focus on routing tables.

What is a Metric?

A metric is a quantitative value used to determine the cost or distance of a particular route. In the context of networking, metrics are used to determine the best path for network traffic to follow based on various factors such as hop count, bandwidth, delay, and load. Routing protocols use different metrics to evaluate the cost of a route, and the lowest cost route is selected as the best path.

How are Metrics Used?

Metrics are used by routing protocols to determine the optimal path for data to travel from one network node to another. Routing protocols calculate the cost of a route based on various metrics, and the route with the lowest cost is selected as the best path. Routing protocols also use metrics to avoid loops and to ensure that traffic is not sent back to the same node from which it originated.

How are Metrics Calculated?

Different routing protocols use different metrics to calculate the cost of a route. The most common routing metrics are:

Hop Count Metric: This metric counts the number of hops, or network devices, that a packet must travel through to reach its destination. The path with the fewest number of hops is considered the best path.

Bandwidth Metric: This metric measures the bandwidth available on a particular route. The path with the highest bandwidth is considered the best path.

Delay Metric: This metric measures the time it takes for a packet to travel from its source to its destination. The path with the lowest delay is considered the best path.

Load Metric: This metric measures the current traffic load on a particular route. The path with the lowest traffic load is considered the best path.

Routing protocols use different algorithms to calculate the cost of a route based on these metrics. For example, OSPF uses a cost metric that is inversely proportional to the bandwidth of a link. The cost of a link is calculated as 10^8 / bandwidth, where bandwidth is measured in bits per second. The lower the bandwidth, the higher the cost of the link.

Comparing Metrics in a Routing Table

Routing tables contain information about the best path to reach a particular network. The information in a routing table includes the destination network address, the subnet mask, the next hop address, the interface used to reach the destination, and the metric used to determine the best path.

Let's take a look at an example routing table:

Destination Network        Subnet Mask            Next Hop        Metric
192.168.1.0                        255.255.255.0        192.168.2.1        2
192.168.2.0                        255.255.255.0        192.168.3.1        1
192.168.3.0                        255.255.255.0        192.168.2.2        3

In this example, we can see that there are three routes to three different networks. Each row contains the destination network address, the subnet mask, the next hop address, and the metric used to determine the best path.

In this example, the route to network 192.168.2.0 has the lowest metric of 1, indicating that it is the best path.

Route Priorities for Different Metrics

Different routing protocols use different metrics to determine the best path for network traffic. The priorities for these metrics can also vary depending on the protocol. Here are some general guidelines for route priorities based on metric values:

For hop count metric:

1. Shorter hop count
2. Lower administrative distance
3. Higher bandwidth
4. Lower delay

For cost metric (used by OSPF):

1. Lower cost
2. Lower administrative distance
3. Higher bandwidth
4. Lower delay

For EIGRP composite metric:

1. Lower composite metric
2. Lower administrative distance
3. Higher bandwidth
4. Lower delay

For BGP:

1. Shorter AS path
2. Higher local preference
3. Lower MED
4. Lower administrative distance

In general, the higher priority is given to the metric that represents the most important aspect for the particular routing protocol. For example, in OSPF, the cost metric is more important than hop count or delay, whereas in BGP, the AS path length is the most important factor. The administrative distance is always considered as a tiebreaker, where the lower administrative distance indicates a higher priority.

Conclusion

In summary, the routing table is a critical component of network routing. It contains information about the network topology, including the routes to different destinations and the metrics associated with each route. Network administrators use this information to ensure that network traffic is routed efficiently and securely. By understanding the components of the routing table, including the network mask, next hop, administrative distance, metric, and prefix length, network administrators can optimize their network routing and troubleshoot issues more effectively.